The invention is related to a modular telecommunication system for providing data exchange between any pair of a multiplicity of data terminals associated with the exchange.
Fully electronic, computer-controlled switching systems for handling digital telecommunications services are already well known. Such switching systems are suitable for use in world-wide telex and gentex networks, set up as public or private data networks or as integrated networks.
A computer-controlled switching system of this type which is known as Siemens system EDX is described in a brochure "Siemens System EDX-System Description", published July 1979 and distributed by Siemens Corp. Iselin, N.J. and which is incorporated herein by reference. The Electronic Data Exchange System described therein consists of modular hardware and software components. The hardware essentially comprises communications hardware, a central processing unit and peripheral equipment. A standard configuration of the exchange system is represented as a block diagram in FIG. 1. The communications hardware connects subscriber lines or trunk lines of a data communication network to the exchange and performs switching-oriented control and through-connect functions. The communications hardware is composed of line terminators LT, terminator group controllers TG and a communications controller CC. Line terminators LT are the electrical interfaces to subscriber lines or trunk lines, and a terminator group controller TG is the logic interface to the communications controller CC. The line terminators LT receive data of various speeds and in different codes, assemble items of data to characters or data words, and present the formatted data to the communications controller CC via a terminator group controller TG.
The communications controller CC is the heart of the communications hardware. It serves as interface between the line terminators LT via the terminator group controller TGC and the central processor CP. The communications controller CC scans up to 4,032 line terminators LT and transfers line status information as well as control information to the central processor CP and control information from the central processor CP to the line terminators LT. The second main function of the communications controller CC is to provide the facilities for transmitting information from one line terminator to another line terminator. The use of this data transfer technique does not load the central processor CP during the call-connect phase and insures a high data through-put rate without internal blocking.
The data exchange system uses central processors of the PDP-11 family of Digital Equipment Corporation for performing the main control functions. The PDP-11/35 model is a 16-bit microprogrammed central processing unit with a central processor bus CP-Bus including bus timing control and bus address registers, a main memory MM, a set of general registers, an internal read-only memory for mircroprograms and an arithmetic logic unit. The central processor controls both communications hardware and device peripherals. For reasons of reliability, the central processor as well as the central parts of the communications hardware and the main peripheral devices are duplicated.
The device peripherals perform auxiliary functions for both central processors and provide storage facilities and man/machine interfaces for system control. These devices are largely concerned with high-speed communication of data to the computer memory. Vital peripherals are, for example, a bus link BL which is a high speed, half duplex data transfer channel connecting the two central processors via the buses. It consists of two general purpose direct memory access (DMA) bus interfaces and transfers words or blocks of data between the central processor memories. Thereby status and control information from the on-line central processor is transmitted to the stand-by central processor and vice-versa. Other peripherals are a console typewriter, a supervisor's teleprinter, disk memories backing up the main memory for storing all on-line programs and buffered call blocks, and input/output peripherals. Programmable Switches PS allow a single peripheral or a group of peripherals to be switched from one bus to another. These programmable switches can be accessed by only one central processor at a time. The programmable switches are designed to disconnect themselves from a bus that is no longer operative and to regenerate all bus signals passing through the switch.
These hardware modules are supported by software modules comprising an operating system and a maintenance system. There are software modules for performing internal coordination, system safeguarding for ensuring interworking between the two computer systems in the exchange with hard stand-by capability for controlling of computer peripherals and of switching-oriented operations. A control system is the central and control oriented part of the operating system, and accomplishes control functions concerning program execution, input/output operations to and from peripheral devices and is responsible for safeguarding. Switching programs perform all functions for switching-oriented operations. They control all phases of a call set-up from the initial call request through the call-connect phase to final clear down and disconnect. Further modules are used program library updating, data conversion between peripheral devices and for other functions such as for program preparation.
When a call is initiated, the coupled line terminator LT detects the call request and sends it via the communications controller CC to the central processor CP. The central processor CP controls the set-up phase and stores all dynamic information necessary for call handling.
The calling party, when enabled, sends selection information which is analyzed by a central processor routine using routing tables to determine an outgoing line to the called party. The central processor CP proceeds in controlling the connection to the called party and switches the call through by entering address information into a memory provided in the communication controller CC. During the following call-connect phase data are transferred from line terminator to line terminator under the control of the communications controller CC. The central processor CP is not involved in handling the data transfer during the call-connect phase.
A call disconnect can be initiated by either party in sending a clearing signal which, is transmitted to the central processor CP. Thereupon, the central processor CP again takes over the control for the clear down of the call. It communicates with both parties involved in that call and invalidates status information in the memory of the communications controller CC. After a guard delay time line terminators LT are left in a condition which allows the lines to be seized at any time.
Evidently, the known system has a distinctive distribution of workload which is shared by line terminators LT, the communications controller CC and the central processor CP. The control of the data exchange system by the microprogrammed central processing system grants a high reliability, since it is implemented as a dual system with hot stand-by capability and offers great flexibility of use with public and private teleprinter and data networks such as terminal exchanges, transit exchanges, international gateway exchanges, and a PABX for direct-connection mode. The configuration of this sytem, however, has one constraint: in a full size configuration the system is restricted to 64 terminator groups, each group consisting of 64 line terminators. This is only the theoretical capacity, since one line terminator LT of each terminator group is reserved for testing purposes of the terminator group, thus the capacity in fact is restricted to 4,032 line terminators LT.
If higher capacity is required, the system can be expanded, as it is schematically indicated in FIG. 1, by an interlink system. This interlink system comprises a system bus S-BUS having an interface arranged between the bus and communications controllers CC of a basic system. The structure of the basic system necessitates to interpret this interface unit as one of the 64 terminator groups which can be associated with the dual communications controllers CC of the basic system. This means that each of both basic systems connected by such an interlink system would be further limited in their capacities and an expanded exchange system composed of two basic systems would have only a theoretical capacity of not more than 7,938 line terminator connections.